This invention relates to zero-current switching, forward power conversion.
One such power converter scheme (e.g., the one described in Vinciarelli, U.S. Pat. No. 4,415,959, issued Nov. 15, 1983, assigned to the same assignee as this application, and incorporated herein by reference) transfers energy from a voltage source for delivery to a load using a transformer that has a controlled amount of effective secondary leakage inductance (e.g. a leakage reactance transformer). On the source side of the transformer, a switch is connected in series with the source and the primary winding of the transformer. The switch connects the source to and disconnects it from the primary winding in a succession of energy transfer cycles. On the load side of the transformer, a first unidirectional conducting device and a capacitor are connected in series with the secondary winding. The capacitor and the effective leakage inductance define a characteristic time scale for the cycling of the switch such that the switch is cycled on and off at times when the current in the switch and the first unidirectional conducting device are essentially zero. The first unidirectional conducting device constrains current flow in the effective leakage inductance to be directed only in the direction of the load, thereby preventing bidirectional energy flow (resonance) from occurring between the effective leakage inductance and the capacitor. Energy is transferred to the load via a second inductor whose value is large in comparison with the effective leakage inductance. This second inductor effectively appears as a current sinking load across the capacitor. A second unidirectional conducting device connected in parallel with the capacitor constrains the capacitor voltage to be unipolar and prevents bidirectional energy transfer from occurring between the second inductor and the capacitor. In the topology so described, the parameters of the circuit elements and the requirement of zero-current switching constrain the converter to unidirectionally transfer a fixed amount of energy during every energy transfer cycle. Because the output power is the product of that fixed amount of energy multiplied by the frequency of occurrence of the energy transfer cycles, control of the output power requires varying the frequency of the energy transfer cycles.
By effectively eliminating switching losses and simultaneously constraining energy transfer to occur only in the forward direction, the prior art topology allows implementation of converters which achieve power densities and efficiencies unachievable with either "non-zero-current switching" topologies or contemporary "resonant" topologies.